Magnetic sensors operate on the principle of detecting magnetic flux density modulation caused by the movement of appropriately configured reluctors (or targets). The magnetic sensor must be affixed very close to the reluctor since its sensitivity decreases very rapidly with the size of the air gap between the reluctor and the magnetic sensor. In most automotive applications, for example, the air gaps are on the order of 0.3 to 1.75 mm. Over such a range of air gaps, the sensor output signal decreases more than ten times. The signal attenuation at large air gaps makes the sensor operation more prone to noise induced failures as well as less accurate in detecting the elements of the reluctor as it spins in relation to the magnetic sensor. Both of these factors are often unacceptable in critical engine control and diagnostic applications.
It may at first glance appear that there would be no problem whatsoever to choose and achieve an appropriate air gap between the magnetic sensor and the reluctor. However, in the majority of production cases, the stack-up of tolerances of the many different components randomly influence the net size of the air gap, which consequently precludes achieving, at each assembly, a precisely predetermined air gap by mere assembly of the parts. As a result, because of the random variations caused by accumulation of tolerances, mere assembly of the parts risks damaging interference between the magnetic sensor and reluctor on the one hand, and inaccurate readings associated with too large an air gap on the other hand. To lessen all the tolerances so that mere assembly assures, at each assembly, the optimum air gap is physically unrealistic and involves inordinate costs associated with manufacturing such precise parts.
The majority of magnetic sensors used in automotive applications involve non-adjustable air gap placement, wherein the stack-up of tolerances causes deviation from the optimal air gap. For example, a rigid bracket is affixed to the body of a magnetic sensor. The magnetic sensor is placed into a sensor port in the engine block, and the bracket is bolted, via a bolt hole in the bracket, to a threaded mounting hole in a mounting surface of the engine block. When the bracket is bolted, the length of the sensor body from the bolt hole of the bracket to the sensor tip determines the air gap with respect to the reluctor, which air gap is affected by the stack-up of tolerances. Even though subject to tolerance related placement inaccuracy, this structural mounting methodology is used widely because of the simplicity of the hardware, and ease of assembly and service.
In situations where air gap variation cannot be tolerated, the air gap is preset during magnetic sensor installation by means of an adjustable bracket, often referred to as a "side mount" bracket. The adjustability of side mount brackets resides in a bolt slot which allows for the bracket to be adjusted along the slot elongation relative to the threaded mounting hole of the mounting surface.
In one form of operation of side mount bracket, the sensor body is placed into the sensor port of the engine block such that the sensor tip is allowed to touch the surface of the reluctor, and then it is withdrawn a distance equal to the predetermined optimum air gap. This method is more time consuming and is error prone.
In another form of operation of side mount bracket, a sacrificial layer of soft abradable material is placed onto the sensor tip, wherein the thickness of the sacrificial layer is equal to the optimum air gap. Now, the installer need merely place the sensor body into the sensor port until the sensor tip touches the reluctor, and then tighten the bolt on the mounting surface with the sensor body retained at this position. During initial rotation of the reluctor, the sacrificial layer may abrade due to reluctor runout or differential thermal expansion without damage being incurred to the sensor body or the reluctor. The sacrificial layer may be either attached to the sensor body or be a part thereof, such as a protuberance, provided the sensor body is of a soft material. However, in the event the magnetic sensor must be re-installed, the abraded sacrificial layer will not be able to again provide position location as it was able to do when unabraded. Therefore, before dismounting the magnetic sensor, the bracket must be marked to indicate the correct position of the sensor body relative to the bracket so that when the new magnetic sensor is re-installed its position on the bracket can be sighted--not an exact procedure. In any event, should the sacrificial layer be exposed to a lubricating oil, the oil spray may carry the abraded debris into oil passageways.
In the prior art, it is known to precisely adjust the air gap using a threaded sensor body and threaded sensor port. This structure is generally used exclusively with magnetic sensors having a single sensing element and having sensing capability unaffected by sensor rotation around its longitudinal axis. In this approach, the sensor tip is brought into touching engagement with the reluctor, and then the sensor body is rotated a predetermined angular amount, wherein the pitch angle of the threads raises the tip a distance equal to the optimum air gap. However, most automotive magnetic sensors contain more than one sensing element and are designed to operate at only one particular angular setting around the sensor axis. Consequently, a threaded sensor body would need to be adjusted in whole revolution steps (ie., 360 degrees) and air gap adjustment would then be in steps of the thread pitch. While the use of a sufficiently small pitch may render the air gap setting resolution adequate, many sensors are precluded from rotation due to geometrical interferences.
Accordingly, what is needed in the art is a structure for holding a magnetic sensor which allows easy and quick re-installation and removal of the magnetic sensor, and provides for automatic setting of an optimal air gap.